I. Field of the Invention
The present invention relates to the in-vivo and in-vitro measurement of glucose-related analytes by spectroscopic means.
II. Description of the Prior Art
Diabetes is a metabolic disorder caused by a body's failure to either produce insulin or use insulin effectively. The hormone, insulin, enables a body to utilize glucose. Approximately 1.4 million Americans suffer from Type I diabetes, where the body fails to produce insulin, and an additional 13 million Americans suffer from type II diabetes, where the body fails to utilize insulin effectively. Currently, seven percent of Americans have diabetes. This percentage is expected to rise to approximately ten percent by the year 2000.
Recent studies have indicated that frequent monitoring and control of blood glucose levels reduces the possibility of serious complications in diabetics by fifty to sixty percent. These studies utilized measurements of the stable glycated hemoglobin complex HbA.sub.1c to evaluate the long-term time average blood glucose level over a several month period immediately preceding the time the sample was taken. The long-term average is based on the approximately 120 day life of the red blood cells containing the glycated hemoglobin. HbA.sub.1c consists of 50 to 90% hemoglobin glycated by a ketoamine linkage at the beta chain N-terminal valine residue. Medical practitioners also use the level of the stable glycated albumin complex fructosamine within the blood serum to provide an evaluation of the approximately 2 to 3 week medium-term time average glucose level of the patient. These two measurements, combined with the immediate blood glucose level, are of great value in diagnosing and treating diabetes. There is medical evidence that the formation of glucose-protein and glucose-lipid complexes may be the direct cause of some of the degenerative effects of diabetes as well as those of aging. In addition to the stable complexes, glucose forms labile complexes with proteins and lipids which are the precursors of the stable complexes. The level of these labile complexes are in equilibrium with the glucose level of the surrounding medium and are therefore highly correlated with the present level of blood glucose. This is particularly true in-vivo because the equilibrium has not been disturbed by the collection or handling of an in-vitro sample. Pre-Hemoglobin A.sub.1c is a labile form of glycated Hb containing glucose bound in aldimine linkage to the beta chain N-terminal valine residue.
The methods currently used to directly measure the levels of stable glycated protein and lipid complexes are invasive in that they require the collection and preparation of a sample for analysis by one of several methods. For example, analysis of the sample may be performed using affinity chromatography, high performance liquid chromatography, ion-exchange chromatography, immunoassay, or calorimetry with thiobarbituric acid (TBA) for total glycated Hb or nitroblue tetrazolium (NBT) for glycated serum albumin. The various methods measure different fractions of the glycated proteins and lipids, some are relatively specific while others measure the total level of several types combined. In addition, they are typically time consuming and labor intensive. Recently, an immunoassay method and bench-top instrumentation for determination of HBA.sub.1c in the physician's office has been introduced (DCA 2000 Hemoglobin A.sub.1c System, Miles, Diagnostics Div.) that improves the speed and ease of analysis although it still requires a blood sample.
In-vitro measurement of the labile fraction of glycated protein and lipids is less common due to the difficulty of maintaining the labile fraction through chemical analysis procedures. In addition, direct in-vitro measurement of blood glucose is well established technology which largely obviates the medical need for in-vitro determination of labile glycated protein and lipid fractions. European Patent Application No. 86200311.8, having publication No. 0 222 419 A2, discloses non-invasive measurement of labile glycosylated hemoglobin in the blood using energy having a visible wavelength range between 520 and 620 nm in order to determine glycemia of a patient. This spectrophotometric method utilizes the shift in wavelength of an absorption peak determined by the ratio of absorption in two semi-areas of the visible spectrum divided at approximately 575 nm. The accuracy and precision of this method is limited by the high absorption of light in this region, which results in a low signal level, and the presence of spectral interferences from other substances present in the blood which cannot be resolved using only two spectral measurements.
One of the invasive methods of directly measuring blood glucose levels requires analysis of a blood sample taken from a patient. The blood sample may be obtained by pricking the patient's finger. The sample is analyzed using chemically treated strips which indicate by color the glucose level in the sample being tested. The strips may be visually compared to color standards or preferably placed in a reader which measures the color reaction on the strip and displays the glucose level. Other enzymatic assays utilize electrochemical or colorimetric measurement techniques. Because these techniques all require at least a finger prick to obtain a blood sample, they are suitable for neither the continuous monitoring nor the repeated testing at frequent intervals which is desirable for the tight control of blood glucose in diabetic patients that has been shown to reduce the long-term medical problems caused by diabetes.
Non-invasive methods and apparatus for direct determination of blood glucose utilizing spectroscopic measurements in the visible (380 to 780 nm) and near-infrared (780 to 2500 nm) regions of the electromagnetic spectrum are disclosed in U.S. Pat. Nos. 4,655,255, 4,882,492, 4,975,581, 5,068,536, 5,077,476, 5,086,229, 5,204,532 and others. To date, none of these direct, in-vivo determinations of blood glucose have reached the accuracy and reliability required for medical use in the management of diabetes. The spectral information utilized by these methods includes not only that generated by glucose in the blood but also that from glucose in the interstitial fluid and other tissues. The spectral information also contains interfering spectral information from the other constituents within the measurement volume.
U.S. Pat. No. 4,975,581, issued to Robinson, et al., discloses a method of and apparatus for determining the similarity of a biological analyte from a model constructed from known biological fluids utilizing a stored model and "an algorithm including (a) all independent sources of said intensity variations v. said wavelengths information from both said set of samples and said biological fluid and (b) more wavelengths than samples . . . "
Pulse oximetry is a related technology which utilizes near-infrared spectroscopy. Two wavelengths in the 700 to 900 nm region of the near-infrared spectrum are used to measure the oxygen saturation of the blood based on the spectral absorption difference between oxy- and deoxy- hemoglobin. The technique utilizes the temporal change in the oxy- and deoxy- hemoglobin absorption measurements caused by the pulsation of the blood pressure to remove steady-state interferences to these measurements.